Aquarium Setup and Care Tips for Anableps anableps

How to care for the four-eyed fish.

The Anableps anableps is an odd fish in more ways than one. It is a surface-dwelling fish that rarely swims around underwater, though it will dive if it feels threatened. It grows to some 12 inches in length, with mature females growing larger than males. Their generally cylindrical body has an olive-brown back. The flanks and belly are a lighter cream color with four narrow parallel dark stripes that begin behind the large pectoral fins decorating the flanks. The most noticeable feature of this fish are the two large, bulbous eyes that protrude from the small head. Observed from above, the eyes and back of this fish continuously break the water line.

Anableps anableps. Photo by Wikipedia

Since just two eyes are visible, where does Anableps anableps’ common name of four-eyed fish come from? Being a surface-dwelling fish, it does have to beware of predators, such as birds from above, as well as larger fish from below. To cope with this, Anableps anableps has large eyes (half of which are submerged as the fish floats on the water surface) that are divided into two parts. The lower half of each eye contains its own pupil, which is adapted for perfect vision underwater, while the upper half of the eye with a second pupil offers clear vision of life above water. Hence, the common name of four-eyed fish is quite relevant. Its above-water vision is excellent for scanning the water surface for insects that fall into the water, which make up its preferred choice of food.

Anableps anableps are livebearers. Not that this is odd in itself, but strangely they are one-sided livebearers. Males have a gonopodium to internally inseminate female fish, and are either right-sided or left-sided, meaning that a right-sided male is only successful when approaching a female from the right-hand side and vice versa. The female fish are also either right-sided or left-sided, as their genital pore is covered with a flap that is hinged on the left side or right side. Since a right-sided male can only inseminate a left-sided female and vice versa, it is important to get male and female fish for the aquarium from one source, just to be on the safe side, if you wish the fish to successfully mate and have offspring. My experience has been that each supplier tends to have compatible males and females.

The four-eyed fish has quite a large range along the Atlantic Coast of Central and South America that even extends to the island of Trinidad. Its favored habitat is made up of the river estuaries and surrounding mangrove swamps, where the water is brackish, with salinity changing with the ebb and flow of the tide, which the four-eyed fish are well able to cope with. It can also sometimes be found in freshwater lagoons along the coast. Groups of mature adult female four-eyed fish tend to swim together in shoals along the water surface, with males following. Here, they usually feed on any terrestrial insects that hit the water surface. Although they are not really noted for diving down to chase food underwater, their diet also includes other aquatic creatures and perhaps small fish, since their large paddlelike pectoral fins give them a good turn of speed when needed.

Aquarium setup

At 12 inches, the four-eyed fish is a comparatively large fish for the aquarium. In order to keep them "happy,” keep them in groups of six or more, and never maintain them singly or even just as pair. They generally spend all their time at the water surface. These considerations dictate that the aquarium for four-eyed fish needs to be at least 48 inches long (90 gallons), but preferably 60 inches long (150 gallons) to accommodate six to eight four-eyed fish, as well as some other fish in the same aquarium. As they are surface-swimming fish, the water level in the aquarium needs to be dropped 12 inches from the top but should be 12 inches deep, dictating an aquarium with a height of 24 inches. Four-eyed fish are good jumpers, so the aquarium needs a tight-fitting cover.

The aquarium bottom should be furnished with a 3-inch gravel bed that can be overlaid with some smooth rocks or even bogwood. If one is to include mudskippers (Periophthalmus spp.) in this setup, the rocks and bogwood would need to break the water surface in one front corner of the aquarium to give a good view of the mudskippers that are normally out of the water most of the time. However, a large water surface should be maintained for the four-eyed fish. It is hard growing regular plants in a brackish aquarium, but one could try some macro algae or even some small mangrove plants that would need regular trimming to keep them within the confines of the aquarium.

While sea water has a specific gravity of around 1.020 to 1.024, brackish water with a specific gravity of around 1.005 is adequate for maintaining four-eyed fish. However, finding a hydrometer calibrated to check this specific gravity level might be difficult. Brackish water can be made by adding 2 level tablespoons of marine salt per gallon of water. Some hobbyists may elect to use aquarium salt or the even cheaper cooking salt, which is just sodium chloride and does not contain all the trace elements found in sea water. The water parameters of a brackish aquarium are generally alkaline and hard (pH: 7.5 to 8.5, dH: 12 to 20), which fortunately can be easily measured.

The brackish aquarium does need a good filtration system, which can be provided by a large external canister filter with a large colony of nitrifying bacteria (Nitrosomonas and Nitrobacter) to convert the fish waste, such as ammonia, first to nitrite and then nitrate. Large fish produce a lot of nitrogenous wastes that can adversely affect the fish if their levels are allowed to build up.

Additionally, regular partial water changes (20 percent) with fresh brackish water are needed every two weeks to remove accumulating nitrates to maintain good water quality. The canister filter return pipe also provides a nice current of water that is necessary in this tank. A heater thermostat or two is needed to maintain the water at a temperature of 77 to 86 degrees Fahrenheit. Adequate lighting can be provided by fluorescent tubes even if mangroves or macro algae are growing in the tank.

The four-eyed fish greatly appreciates live insects taken at the water surface. The garden is a great provider of insects and other crustaceans of every variety, though the easiest to collect are probably woodlice. Lifting any old piece of wood in the garden and lightly tapping it inside a bowl dislodges a multitude of different-sized woodlice that are appropriate food for these fish. These woodlice are crustaceans that have properly invaded land without the need to return to water in order to breed. Because they feed on dead leaves and stems found in the soil, there are hundreds of these half-inch bugs around compost heaps.

Fortunately, four-eyed fish will also feed on floating food sticks, pellets, and even flake and frozen foods. All these various types of foods are generally taken at or near the water surface by the four-eyed fish and not at substrate level, while sailfin mollies and orange chromides will happily take food mid-water and off the substrate.

Tankmates

Other fish for a brackish tank containing at least six four-eyed fish could be a group of four archerfish (Toxotes jaculatrix) or monos (Monodactylus argenteus). However, these fish also grow quite large, reaching 6 inches or more at maturity and are not suitable for an aquarium that has just a 12-inch depth of water. Their minimum depth should be at least 24 inches. Alternative aquarium choices for the brackish aquarium are some colorful sailfin mollies or even a group of orange chromide cichlids (Etroplus maculatus), both of which have completely different lifestyles to the four-eyed fish and also occupy a different niche of the aquarium. Also, these smaller fish put less of a burden on the filtration system and should do all right in the shallow water in this half-filled aquarium. Mudskippers (Periophthalmus spp.) would also make an interesting addition to this setup, as they spend a lot of time out of the water on the exposed rocks or bogwood above the water line, but they may need some smaller live food to keep them healthy.

Breeding Four-eyed Fish

Four-eyed fish will usually mate and produce live young without the need for any special setups, provided the fish are sexually mature and include healthy males and females that are sexually compatible with each other. Four-eyed fish are mature by the time they reach 8 months of age, at which point they should have reached more than 6 inches in length. As mentioned, four-eyed fish are right-sided or left-sided when it comes to breeding, and therefore compatible pairs are needed to successfully breed.

The four-eyed fish belong to the family Anablepid, which are classified as viviparous fish (livebearers) in which developing embryos undergo follicular gestation (embryo develops in the ovarian follicle) inside the female fish.

Here, each developing embryo is nourished principally through the embryo’s expanded belly sac, the surface of which is covered with delicate blood vessels, which can absorb nutrients and oxygen from the mother’s blood supply, while wastes, metabolites and carbon dioxide from the embryo are carried away by the mother’s blood supply, just as in humans. In the case of premature release of the fry before development is complete, this delicate belly sac area can rupture, leading to the fry’s gut protruding out of this opening in its belly. Sometimes, the fry may survive, with development continuing after their premature release if water conditions are perfect; but often these fry will perish.

The gestation period of about eight weeks is dependent on the temperature of the water and usually results in 10 to 15 large fry that are already 2 to 2.8 inches when the female drops them, usually tail first. The fry, which are miniature copies of the female fish, may be eaten by other fish in the aquarium, so they should be moved to their own aquarium that contains similar conditions to the main aquarium. The fry will grow quickly if they are well-fed with small live food, such as fruit flies, blood worms and Daphnia, as well as flakes and small floating pellets. Male and female young four-eyed fish are initially identical, but eventually the male’s anal fin develops into a gonopodium, while females grow faster and are eventually larger than males. The ratio of male to female fry is usually one to one.

A Last Look

Four-eyed fish are robust fish that are easy to maintain in an aquarium that is large enough to accommodate a group of six or more. With smaller groups, it might be best to have just one male with two or three females of the same orientation that are sexually compatible with the male. With just a pair, the male’s continuous attention to just the one female can be a bit of a harassing nuisance.

Four-eyed fish are easy to feed, as they will accept floating pellets readily in lieu of live crustaceans, though live food is readily available in any garden. Their unusual and interesting lifestyle at the water surface is best observed in an aquarium only half filled with water. They are also easy to breed, and with their longer gestation period of two months leading to larger fry that are easy to care for, this virtually guarantees success.

Four-eyed fish do best in brackish aquariums, but do not let this put you off as they are easily set-up; only a relatively small amount of salt needs to be added to the aquarium water. Maintenance is similar to that of a freshwater aquarium and does not need all the expensive equipment needed for a marine tank containing delicate fish and corals. Four-eyed fish in a brackish aquarium will add a whole new dimension to your fishkeeping hobby.

Iggy Tavares has been keeping fish since the early 1960s. He studies and breeds popular freshwater fish; his passion is breeding, photographing and writing about cichlids.

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